Hydrodynamic bearing device and disk rotating apparatus

ABSTRACT

In a hydrodynamic bearing device in which a radial bearing face having a dynamic pressure generating groove on a shaft or an inner periphery of a sleeve is provided and a clearance between the shaft and the sleeve is filled with lubricant, an annular depression is provided on one end face of the sleeve adjacent to a rotor hub and a cover plate for covering the depression is attached to the sleeve so as to define a reservoir for the lubricant or air for the purpose of preventing such a risk that absence of an oil film occurs in clearances of a bearing of the hydrodynamic bearing device due to outflow of oil upon forcing of the oil by air received into the bearing. A step portion is provided on the other end face of the sleeve such that the step portion and the reservoir are communicated with each other by a communication hole. During operation of the hydrodynamic bearing device, air in the hydrodynamic bearing device reaches the reservoir via the communication hole so as to be discharged from the reservoir.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/554,473 filed on May 26, 2006, which is a national stage ofInternational Application No. PCT/JP2004/005277 filed Apr. 13, 2004.This application claims the benefit and priority of JP 2003-120025 filedApr. 24, 2003 and JP 2003-176481 filed Jun. 20, 2003. The entiredisclosure of each of the above applications are incorporated herein byreference.

BACKGROUND ART

In recent rotational recording apparatuses employing a magnetic disk,etc., data-transfer velocity is rising upon increase of their storagecapacity. Hence, since a disk rotating apparatus employed in suchrecording apparatuses requires high-speed and high precision rotations,a hydrodynamic bearing device is used in a rotary main shaft portion ofthe disk rotating apparatus.

Hereinafter, a conventional hydrodynamic bearing device is describedwith reference to FIGS. 18 and 19. In FIG. 18, a shaft 31 is rotatablyinserted into a bearing bore 32A of a sleeve 32 mounted on a base 35. InFIG. 18, the shaft 31 has a flange 33 formed integrally at its lower endportion. The flange is received in a step portion 32K of the sleeve 32so as to rotatably confront a thrust plate 34. A rotor hub 36 to which arotor magnet 38 is secured to is attached to the shaft 31. A pluralityof disk 39 held by a spacer 40 and a clamper 41 are mounted on the rotorhub 36. A motor stator 37 confronting the rotor magnet 38 is mounted onthe base 35. Dynamic pressure generating grooves 32B and 32C areprovided on an inner peripheral surface of the bearing bore 32A of thesleeve 32. Dynamic pressure generating grooves 33A are provided on oneface of the flange 33, which confronts the step portion 32K of thesleeve 32, while dynamic pressure generating grooves 33B are provided onthe other face of the flange 33, which confronts the thrust plate 34.Clearances between the shaft 31 and the flange 33 on one hand and thesleeve 32 on the other hand, which include the dynamic pressuregenerating grooves 32B, 32C, 33A and 33B, are filled with oil 42. One ormore vent holes 32E are provided on the sleeve 32 substantially inparallel with an axis of the sleeve 32. A lower end of the vent holes32E communicates with a space which is disposed at a lower end portionof the sleeve 32 so as to contain the flange 33. An upper end of thevent holes 32E opens to an upper end face of the sleeve 32.

Operation of the conventional hydrodynamic bearing device of the abovedescribed arrangement is described by referring to FIGS. 18 and 19. InFIG. 18, if the motor stator 37 is energized, a rotary magnetic field isgenerated and thus, the rotor magnet 38, the rotor hub 36, the shaft 31and the flange 33 start rotations. At this time, a pumping pressure isgenerated in the oil 42 by the dynamic pressure generating grooves 32B,32C, 33A and 33B. Thus, the shaft 31 is raised and is rotated withoutcoming into contact with the thrust plate 34 and the inner peripheralsurface of the bearing bore 32A while being lubricated by the oil 42. Amagnetic head (not shown) is brought into contact with the disks 39 soas to perform recording and reproduction of electrical signals.

The above conventional hydrodynamic bearing device has the followingproblems. FIG. 19 is a fragmentary sectional view including the shaft 31and the sleeve 32 of FIG. 18. As shown in FIG. 19, the shaft 31 isrotated in the bearing bore 32A of the sleeve 32 while being lubricatedby the oil 42. When the hydrodynamic bearing device has been assembledor while the hydrodynamic bearing device is being transported, air lumpsor air bubbles (hereinafter, referred to as “air 43A or 43B”) maypenetrate into the oil 42 in the bearing bore 32A. For example, in caseambient pressure has changed during transport in an aircraft,penetration of air bubbles may happen. If volume of the air 43Apenetrating into the vicinity of the dynamic pressure generating grooves32B and 32 c is expanded by rise of temperature or drop of atmosphericpressure, a portion of the dynamic pressure generating grooves 32 b iscovered by air, thereby resulting in absence of the oil film. Meanwhile,a portion of the oil may leak out of the hydrodynamic bearing device asindicated by oil 42B. Meanwhile, if the air 43B penetrating into thevicinity of the flange 33 is expanded, the hatched oil 42A in the venthole 32E may be pushed upwardly by expanded air 43C so as to leak out ofthe hydrodynamic bearing device as shown by oil 42D. If the oil 42 leaksoutwardly, shortage of quantity of the oil in the bearing occurs. As aresult, there is a risk of extreme aggravation of reliability due tocontact of the shaft 31 with the sleeve 32 during rotation.

Meanwhile, also in case a drop impact load (acceleration) is applied tothe conventional hydrodynamic bearing device in the direction of thearrow G1 as shown in FIG. 19, there is a risk that the oil 42 leaksoutwardly as shown by the oil 42B.

DISCLOSURE OF INVENTION

The present invention has for its object to provide a hydrodynamicbearing device which is highly reliable by preventing lubricant such asoil filled in the hydrodynamic bearing device from leaking out of abearing, and a disk rotating apparatus including the hydrodynamicbearing device.

A hydrodynamic bearing device of the present invention includes a sleevehaving a bearing bore into which a shaft is rotatably inserted and acover plate which is provided such that a reservoir for storinglubricant and air is defined in the vicinity of one end portion of thebearing bore. A substantially disklike flange is secured to one endportion of the shaft and has one face confronting one end face of thesleeve in the vicinity of the other end portion of the bearing bore. Athrust plate is provided so as to confront the other face of the flangeand seal a region including the one end face of the sleeve. Acommunication path is formed for establishing communication between thereservoir and the region. First and second dynamic pressure generatinggrooves of a herringbone pattern are arranged in a direction along anaxis of the shaft on at least one of an inner peripheral surface of thebearing bore of the sleeve and an outer peripheral surface of the shaft.A third dynamic pressure generating groove of a herringbone pattern isprovided on at least one of opposed faces of the flange and the thrustplate and a fourth dynamic pressure generating groove of a herringbonepattern is provided on at least one of the one face of the flange andthe one end face of the sleeve. Clearances between the shaft and thesleeve and between the flange and the thrust plate including the first,second, third and fourth dynamic pressure generating grooves are filledwith lubricant. One of the sleeve and the shaft is mounted on astationary base and the other of the sleeve and the shaft is mounted ona rotary member.

In the present invention, the reservoir and the covered region arecommunicated with each other by the communication path. Thus, thelubricant filled in the clearance between the shaft and the bearing boreof the sleeve is circulated by way of the communication path duringoperation of the hydrodynamic bearing device. By circulation of thelubricant, air such as air bubbles mixed into the lubricant is alsocirculated together with the lubricant. When the air bubbles containedin the lubricant have reached the reservoir during the circulation, theair bubbles are separated from the lubricant and are dischargedoutwardly. Since the reservoir is covered by the cover plate, thelubricant does not leak outwardly. Since the air in the lubricant isautomatically removed during operation of the hydrodynamic bearingdevice in this way, air mixed into the lubricant during assembly of thehydrodynamic bearing device is also removed gradually and thus, only thelubricant is left in the hydrodynamic bearing device. The lubricantflows into the clearance between the shaft and the sleeve from thereservoir but does not leak outwardly. Hence, shortage of the lubricantor absence of the oil film does not occur between the shaft and thesleeve and thus, the hydrodynamic bearing device operates stably.Accordingly, the hydrodynamic bearing device having high long-termreliability can be materialized.

A hydrodynamic bearing device in another aspect of the present inventionincludes a shaft which has, at its one end portion, a thrust bearingface perpendicular to an axis of the shaft and a sleeve having a bearingbore into which the shaft is rotatably inserted such that the bearingbore acts as a radial bearing. A cover plate is provided such that areservoir for storing lubricant and air is defined in the vicinity ofone end portion of the bearing bore. A thrust plate is provided so as toseal the other end portion of the bearing bore and confront the thrustbearing face of the shaft. A communication path is formed forestablishing communication between the reservoir and a region of theother end portion of the bearing bore. First and second dynamic pressuregenerating grooves of a herringbone pattern are arranged in a directionalong the axis of the shaft on at least one of an inner peripheralsurface of the bearing bore of the sleeve and an outer peripheralsurface of the shaft. A third dynamic pressure generating groove of aherringbone pattern is provided on at least one of the thrust bearingface and one face of the thrust plate confronting the thrust bearingface. Clearances between the shaft and the sleeve and between the thrustbearing face and the thrust plate including the first, second and thirddynamic pressure generating grooves are filled with lubricant. One ofthe sleeve and the shaft is mounted on a stationary base and the otherof the sleeve and the shaft is mounted on a rotary member.

In the present invention, the reservoir and the covered region arecommunicated with each other by the communication path. Thus, thelubricant filled in the clearance between the shaft and the bearing boreof the sleeve is circulated by way of the communication path duringoperation of the hydrodynamic bearing device. By circulation of thelubricant, air such as air bubbles mixed into the lubricant is alsocirculated together with the lubricant. When the air bubbles containedin the lubricant have reached the reservoir during the circulation, theair bubbles are separated from the lubricant and are dischargedoutwardly. Since the air in the lubricant is automatically removedduring operation of the hydrodynamic bearing device in this way, airmixed into the lubricant during assembly of the hydrodynamic bearingdevice is also removed gradually and thus, only the lubricant is left inthe hydrodynamic bearing device. The lubricant flows into the clearancebetween the shaft and the sleeve from the reservoir but does not leakoutwardly. Hence, shortage of the lubricant or absence of the oil filmdoes not occur between the shaft and the sleeve and thus, thehydrodynamic bearing device operates stably. Accordingly, thehydrodynamic bearing device having high long-term reliability can bematerialized.

In the present invention, since the third dynamic pressure generatinggroove is provided on the thrust bearing face of the shaft so as to forma thrust bearing portion, the construction is simplified without theneed for provision of the flange.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a hydrodynamic bearing device according toa first embodiment of the present invention.

FIG. 2 is an enlarged fragmentary sectional view showing a shaft and asleeve of the hydrodynamic bearing device of the first embodiment of thefirst embodiment.

FIG. 3 is a top plan view of a flange 3 of the hydrodynamic bearingdevice of the first embodiment of the present invention.

FIG. 4 is a bottom plan view of the flange 3 of the hydrodynamic bearingdevice of the first embodiment of the present invention.

FIG. 5 is a fragmentary sectional view showing an operation of thehydrodynamic bearing device of the first embodiment of the presentinvention including the shaft and the sleeve.

FIG. 6 is a fragmentary sectional view showing a further operation ofthe hydrodynamic bearing device of the first embodiment of the presentinvention including the shaft and the sleeve.

FIG. 7 is a fragmentary sectional view showing a still further operationof the hydrodynamic bearing device of the first embodiment of thepresent invention including the shaft and the sleeve.

FIG. 8 is a fragmentary sectional view showing a shaft and a sleeve of ahydrodynamic bearing device according to a second embodiment of thepresent invention.

FIG. 9 is a fragmentary sectional view showing a shaft and a sleeve of ahydrodynamic bearing device according to a third embodiment of thepresent invention.

FIG. 10 is a fragmentary sectional view showing a shaft and a sleeve ofa hydrodynamic bearing device according to a fourth embodiment of thepresent invention.

FIG. 11 is a fragmentary sectional view showing a shaft and a sleeve ofa hydrodynamic bearing device according to a fifth embodiment of thepresent invention.

FIG. 12 a is a perspective view of a cover plate of the hydrodynamicbearing device of the fifth embodiment of the present invention.

FIG. 12 b is a sectional view of the cover plate of the hydrodynamicbearing device of the fifth embodiment of the present invention.

FIG. 13 is a fragmentary sectional view showing a shaft and a sleeve ofa hydrodynamic bearing device according to a sixth embodiment of thepresent invention.

FIG. 14 a is a perspective view of a cover plate of the hydrodynamicbearing device of the sixth embodiment of the present invention.

FIG. 14 b is a sectional view of the cover plate of the hydrodynamicbearing device of the sixth embodiment of the present invention.

FIG. 15 is a perspective view of a further cover plate of thehydrodynamic bearing device of the sixth embodiment of the presentinvention.

FIG. 16 is a fragmentary sectional view showing a shaft and a sleeve ofa hydrodynamic bearing device according to a seventh embodiment of thepresent invention.

FIG. 17 is a sectional view showing a clearance of a radial bearing ofthe hydrodynamic bearing device of the seventh embodiment of the presentinvention.

FIG. 18 is a sectional view of a prior art hydrodynamic bearing device.

FIG. 19 is a fragmentary sectional view showing a shaft and sleeve ofthe prior art hydrodynamic bearing device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of a hydrodynamic bearing device anda disk rotating apparatus having the hydrodynamic bearing device in thepresent invention are described with reference to FIGS. 1 to 17.

First Embodiment

A hydrodynamic bearing device according to a first embodiment of thepresent invention is described with reference to FIGS. 1 to 7. FIG. 1 isa sectional view of the hydrodynamic bearing device of the firstembodiment of the present invention, while FIG. 2 is an enlargedfragmentary sectional view showing a shaft 1 and a sleeve 2. In FIG. 1,the sleeve 2 has a bearing bore 2A and the cylindrical shaft 1 isrotatably inserted into the bearing bore 2A. There is a minute clearancebetween an outer peripheral surface of the shaft 1 and an innerperipheral surface of the bearing bore 2A of the sleeve 2. Dynamicpressure generating grooves 1B and 1C of a known herringbone pattern inwhich each groove is bent at an angular portion are formed on at leastone of the outer peripheral surface of the shaft 1 and the innerperipheral surface of the bearing bore 2A of the sleeve 2 so as to actas a radial bearing portion. The radial bearing portion supports theshaft 1 in a radial direction of the shaft 1. In the example of FIG. 1,the dynamic pressure generating grooves 1B and 1C are formed on theinner peripheral surface of the bearing bore 2A. Each of the dynamicpressure generating grooves 1B and 1C has the herringbone pattern. InFIG. 1, at least one of the dynamic pressure generating grooves 1B and1C (the dynamic pressure generating grooves 1B in the example of FIG. 1)are formed such that a length of a lower groove 1M from an angularportion 1K is smaller than that of an upper groove 1L from the angularportion 1K as shown in FIG. 2.

In FIG. 1, a rotor hub 12 having a rotor magnet 8 is mounted on an upperend of the shaft 1. A flange 3 having faces orthogonal to an axis of theshaft 1 and a diameter larger than that of the shaft 1 is integrallyformed at a lower end of the shaft 1. A thrust bearing face 3F disposedat the lower face of the flange 3 confronts a thrust plate 4 fixed tothe sleeve 2. The thrust plate 4 seals an end portion region of thethrust bore 2A of the sleeve 2, which includes the flange 3. Dynamicpressure generating grooves 3B of a helical shape or a herringbonepattern are formed on one of a lower face of the flange 3 and an upperface of the thrust plate 4 (the lower face of the flange 3 in FIG. 1) soas to act as a thrust bearing portion.

Dynamic pressure generating grooves 3A are formed on one of an outerperipheral portion of an upper face of the flange 3 and a step portion2D of the sleeve 2, which confronts the outer peripheral portion of theupper face of the flange 3 (the upper face of the flange 3 in FIG. 1). Aknown large clearance portion 2B is formed at an axially intermediateportion of the bearing bore 2A of the sleeve 2 but is not relevant tothe present invention directly, so that its description is abbreviated.The flange 3 is received in the step portion 2D of the sleeve 2. Aclearance or a recess 3C for storing oil is provided on the lower faceof the flange 3.

An annular upper depression 2C surrounding the bearing bore 2A isprovided on an upper end face of the sleeve 2. A ringlike cover plate 5is attached to the sleeve 2 so as to cover the upper depression 2C. Anouter peripheral portion of the cover plate 5 is secured to an outerperipheral portion of the sleeve 2 by caulking or the like. An innerperipheral portion of the cover plate 5 is mounted so as to hold a smallclearance with an upper end portion of the bearing bore 2A of the sleeve2 as will be described later in detail. A space or a clearance definedby the upper depression 2C and the cover plate 5 is referred to as anupper reservoir 15. Oil is stored in the upper reservoir 15 asnecessary. In the upper reservoir 15, dimension of the clearanceinterposed between the cover plate 5 and the sleeve 2 is not constant inthe radial direction. Namely, the dimension is made sufficiently smallat the opening 15A confronting the outer peripheral surface of the shaft1, i.e., at an inner peripheral portion of the upper reservoir 15 and ismade large at an outer peripheral portion of the upper reservoir 15.

A first communication hole 2E extending substantially in parallel withan axis of the bearing bore 2A is provided on the sleeve 2. The firstcommunication hole 2E communicates, at its upper end, with the upperreservoir 15 and communicates, at its lower end, with a space includingthe step portion 2D of the sleeve 2 so as to form a communication path.The sleeve 2 is secured to a base 6 on which a motor stator 7 ismounted. A gap between the shaft 1 and the bearing bore 2A of the sleeve2 including a clearance between the shaft 1 and the sleeve 2 and aclearance between the flange 3 and the thrust plate 4 is filled withlubricant (hereinafter, referred to as “oil”) 13. Since the oil 13 has acertain viscosity, air bubbles 14 may penetrate in between the shaft 1and the bearing bore 2A as shown in FIG. 2. The oil enters also thefirst communication hole 2E and the upper reservoir 15 but a smallamount of air (air bubbles) 14 are present in the first communicationhole 2E and the upper reservoir 15. As shown in FIG. 1, a plurality ofdisks 9 are mounted on the rotor hub 12 by a spacer 10 and a clamper 11such that a disk rotating apparatus is constituted.

Operation of the hydrodynamic bearing device of the above describedarrangement is described with reference to FIGS. 1 to 7, hereinafter. InFIG. 1, if the motor stator 7 is energized from a power source (notshown), a rotary magnetic field is generated and thus, the rotor hub 12on which the rotor magnet 7 is mounted starts rotation together with theshaft 1, the flange 3, the disks 9, the clamper 11 and the spacer 10. Ifthe rotation is started, the dynamic pressure generating grooves 1B, 1C,3A and 3B collect the oil 13 to predetermined locations so as togenerate a known pumping pressure. Hence, the shaft 1 is raised and isrotated at high speed without coming into contact with the sleeve 2 andthe thrust plate 4. FIG. 2 shows a state in which the air 14 mixes intothe oil 13 during rotation of the hydrodynamic bearing device.

FIG. 3 is a top plan view showing an example of the known dynamicpressure generating grooves 3A provided on the upper face of the flange3, which confront the step portion 2D of the sleeve 2. FIG. 4 is a topplan view showing an example of the known dynamic pressure generatinggrooves 3B provided on the lower face of the flange 3. The radially bentdynamic pressure generating grooves 3A and 3B shown in FIGS. 3 and 4collect the oil 13 so as to generate a thrust force parallel to the axisof the shaft 1.

FIG. 5 is an enlarged fragmentary sectional view showing the shaft 1 andthe sleeve 2 of the hydrodynamic bearing device of this embodiment. InFIG. 5, “S1” denotes a dimension of a radial clearance of the dynamicpressure generating grooves 1B and “S2” denotes a dimension of a radialclearance between the outer periphery of the shaft 1 and the cover plate5. An upper end portion 2H of the bearing bore 2A of the sleeve 2 has adiameter larger than that of the bearing bore 2A. The “dimension of theradial clearance” is defined as a dimension of a clearance between theouter periphery of the shaft 1 and the inner periphery of the bearingbore 2A at the time the axis of the shaft 1 is held in alignment with acentral axis of the bearing bore 2A of the sleeve 2. “S3” denotes adimension of the clearance 15A of a portion of the upper reservoir 15confronting the shaft 1, i.e., an inner peripheral portion. “S4” denotesa dimension of a clearance at an outer peripheral portion of the upperreservoir 15. In this embodiment, the dimensions S1 and S2 of the radialclearances and the dimensions S3 and S4 of the clearances are set so asto have the following relations.

S1<S2, S1<S3 and S3<S4

By setting the clearances as described above, the oil 13 stored in theupper reservoir 15 moves, by its surface tension, to the neighborhood ofthe opening 15A of the clearance dimension S3 smaller than the clearancedimension S4. From the opening 15A of the clearance dimension S3, theoil 13 further enters the smaller radial clearance of the dimension S1between the shaft 1 and the bearing bore 2A so as to flow, as shown bythe arrow 13A, to a region of the dynamic pressure generating grooves 1Bacting as the radial bearing portion.

In the dynamic pressure generating grooves 1B and 1C of the radialbearing, each of the dynamic pressure generating grooves 1B has an uppergroove 1L and a lower groove 1M from an angular portion 1K and a lengthL of the upper groove 1L is made larger than a length M of the lowergroove 1M such that a vertically asymmetric herringbone pattern isformed. Therefore, the oil 13 flowing into the radial clearance of thedimension S2 between the shaft 1 and the upper end portion 2H of thebearing bore 2A is sucked, by pumping action at the time of start of thehydrodynamic bearing device and during rotation of the hydrodynamicbearing device, into the radial bearing between the bearing bore 2A andthe shaft 1 including the dynamic pressure generating grooves 1B and 1C.Thus, the oil 13 present in the upper reservoir 15 flows into the radialbearing as shown by the arrow 13A. As a result, in the clearance betweenthe shaft 1 and the bearing bore 2A, the oil 13 flows in a directionindicated by the arrow 13C. Therefore, the oil 13 disposed adjacent tothe flange 3 is delivered so as to flow into the communication hole 2Eand reaches the upper reservoir 15. Then, the oil 13 again flows fromthe opening 15A between the cover plate 5 and the sleeve 2 into theradial bearing portion between the shaft 1 and the bearing bore 2A so asto circulate through the hydrodynamic bearing device. By circulation ofthe oil 13, the air bubbles 14 in the oil 13 also passes through thecommunication hole 2E together with the oil 13 so as to reach the upperreservoir 15. The air bubbles 14 which have reached the upper reservoir15 are discharged outwardly from the clearance between the cover plate 5and the sleeve 2.

Discharge of air is described in more detail with reference to FIG. 6.FIG. 6 is a fragmentary sectional view showing state of air which hasentered the oil 13 in the hydrodynamic bearing device. If quantity ofair 14A formed by air bubbles or air lumps present in the hydrodynamicbearing device increases, internal pressure of the air 14A rises uponrise of ambient temperature or the air 14A is expanded by drop ofatmospheric pressure, volume of the air 14A increases. In such a case,the air 14A enters the first communication hole 2E from a lower inlet 2Fof the first communication hole 2E and proceeds upwardly together withthe oil 13 as indicated by air 14D. The air 14D which has reached anupper end 2G of the first communication hole 2E enters the upperreservoir 15 and is discharged outwardly from the small clearancebetween the cover plate 5 and the sleeve 2 as shown by the arrow C. Inthe first communication hole 2E, the oil 13 also proceeds upwardlytogether with the air 14D. However, after the oil 13 has been carried tothe upper reservoir 15, the oil 13 remains in the upper reservoir 15 dueto its surface tension. Thus, only the air 14D is discharged. Therefore,since the oil 13 is neither forced nor leaked out of the hydrodynamicbearing device, such a phenomenon that absence of the oil film is causedby shortage of the oil 13 does not happen, so that the hydrodynamicbearing device is rotated stably.

In a concrete example of the embodiment shown in FIG. 5, the shaft 1 hasa diameter of 1 to 20 mm. The clearance dimension S3 ranges from 30 to150 microns. The dimension S1 of the radial clearance of the radialbearing ranges from 1 to 10 microns. The first communication hole 2E hasa diameter of 0.3 to 1.0 mm. Experiments conducted by the presentinventors have revealed that if the diameter of the shaft 1, theclearance dimension S3, the dimension S1 of the radial clearance and thediameter of the first communication hole 2E fall in the respectiveranges referred to above, the oil 13 is held in the hydrodynamic bearingdevice without leaking outwardly and only the air 14 is dischargedoutwardly.

As shown in FIG. 7 which is a fragmentary sectional view similar to FIG.6, the present inventors have made various tests in which a drop impactload or vibrations are applied in a direction of the arrow G2. The testresults have shown that the oil 13 stored in the upper reservoir 15 isheld in the upper reservoir 15 due to its surface tension withoutflowing out of the hydrodynamic bearing device. In the experiments, ithas been found that even if an acceleration of 2,500 G is applied to thehydrodynamic bearing device for 1 to 10 msec by setting both of theclearance dimensions S2 and S3 to about 50 microns, the oil 13 does notleak.

In this embodiment, air such as air bubbles, which has entered the oil13 in the hydrodynamic bearing device, proceeds to the upper reservoir15 of the sleeve 2 by way of the first communication hole 2E duringoperation of the hydrodynamic bearing device and is discharged out ofthe hydrodynamic bearing device therefrom. However, the oil 13 remainsin the upper reservoir 15 without leaking outwardly. For example, sinceair which has entered the oil during manufacture of the hydrodynamicbearing device is also removed during use of the hydrodynamic bearingdevice, long-term reliability of the hydrodynamic bearing device isupgraded. Meanwhile, the single first communication hole 2E isillustrated in FIG. 1 but a plurality of the first communication holes2E may be provided on the sleeve 2. As shown by the dotted lines in FIG.6, a communication hole 2Q for establishing communication between theupper reservoir 15 and space of the step portion 2D may be providedbetween the outer periphery of the sleeve 2 and the base 6 in place ofthe first communication hole 2E. In this case, a vertical groove may beprovided on the outer periphery of the sleeve 2 as a portion of thecommunication hole 2Q corresponding to the outer peripheral portion ofthe sleeve 2.

Second Embodiment

FIG. 8 is a fragmentary sectional view showing the shaft 1 and a sleeve20 of a hydrodynamic bearing device according to a second embodiment ofthe present invention. In FIG. 8, a second communication hole 20J forestablishing communication between the first communication hole 2E and alarge clearance portion 20B is provided at a central portion of thesleeve 20. Other constructions of this hydrodynamic bearing device aresimilar to those of the hydrodynamic bearing device of the firstembodiment shown in FIG. 1.

In order to form the second communication hole 20J, there is, forexample, a method in which as shown in FIG. 8, a hole is formed on thesleeve 20 in a direction of the arrow 20H with a drill. After the holehas been formed on the sleeve 20, a hole 20K on the outer periphery ofthe sleeve 20 is sealed with a plug 17.

In the hydrodynamic bearing device of this embodiment, the firstcommunication hole 2E communicates with space between the dynamicpressure generating grooves 1B and the dynamic pressure generatinggrooves 1C via the second communication hole 20J. Thus, the oil 13 flowsfrom the upper reservoir 15 into a portion of the dynamic pressuregenerating grooves 1B as shown by the arrow 13A and flows also from thesecond communication hole 20J into the portion of the dynamic pressuregenerating grooves 1B as shown by the arrow 13D. The oil 13 which hasflowed into the portion of the dynamic pressure generating grooves 1B inthe direction of the arrow 13D flows together with the oil 13 havingflowed into the portion of the dynamic pressure generating grooves 1B inthe direction of the arrow 13A and returns from a lower inlet 20F to thefirst communication hole 2E through a space between the shaft 1 and thebearing bore 20A including the dynamic pressure generating grooves 1Band 1C. Air mixed into the oil 13 is separated from the oil 13 when theoil 13 flows into the second communication hole 20J as shown by thearrow 13G. The separated air 14 proceeds in a direction of the arrow 14Fand is discharged outwardly by way of the upper reservoir 15.

In this embodiment, since the oil 13 is displaced vigorously byproviding the second communication hole 20J, removal of the air 14 fromthe oil 13 is performed efficiently. As a result, reliability of thehydrodynamic bearing device is upgraded further. Meanwhile, even if theair 14 has entered the oil 13 in the hydrodynamic bearing device forsome reason, the air 14 is discharged out of the hydrodynamic bearingdevice rapidly, so that reliability of the hydrodynamic bearing devicebecomes high.

Third Embodiment

FIG. 9 is a fragmentary sectional view showing a shaft 30 and the sleeve2 of a hydrodynamic bearing device according to a third embodiment ofthe present invention. In FIG. 9, a small diameter portion 30A having adiameter smaller than that of the shaft 30 is provided on the shaft 30in the vicinity of an end portion of the shaft 30 coupled with the rotorhub 12. A diameter of an inner peripheral edge 25A of a ringlike coverplate 25 is larger than that of the small diameter portion 30A but issmaller than that of the shaft 30. Namely, the cover plate 25 isarranged to cover the clearance between the shaft 30 and the sleeve 2.Other constructions of this hydrodynamic bearing device are similar tothose of the first embodiment shown in FIG. 1. By this arrangement, itis possible to further positively prevent outward leakage of the oil 13from the clearance between the shaft 30 and the cover plate 25.Meanwhile, since the diameter of the inner peripheral edge 25A of thecover plate 25 is smaller than that of the shaft 30, the shaft 30 is notdetached from the bearing bore 2A of the sleeve 2. Namely, the coverplate 25 functions to prevent detachment of the shaft 30.

In FIG. 9, it is supposed that “S1” denotes a dimension of a radialclearance in the neighborhood of the dynamic pressure generating grooves1B and “S2” denotes a dimension of a radial clearance between the shaft30 and the upper end portion 2H of the sleeve 2. The upper end portion2H of the bearing bore 2A of the sleeve 2 has a diameter lager than thatof the bearing bore 2A. In the upper reservoir 15 defined by the coverplate 25 and the upper depression 2C of the sleeve 2, it is supposedthat “S3” denotes a dimension of a clearance of an inner peripheralportion of the upper reservoir 15 and “S5” denotes a dimension of anaxial clearance between the cover plate 25 and an upper end of the shaft30. It is supposed that “S6” denotes a dimension of a radial clearancebetween the small diameter portion 30A of the shaft 30 and the innerperipheral edge 25A of the cover plate 25. In this embodiment, thedimension S1 is set to be smaller than the dimensions S2, S3, S5 and S6,i.e., S1<S2, S1<S3, S1<S5 and S1<S6. Oil has a property to flow into asmallest clearance by its surface tension. Thus, by setting thedimensions S1, S2, S3, S5 and S6 as described above, the oil 13 storedin the upper reservoir 15 flows into the clearance of the smallestdimension S1 between the shaft 30 and the bearing bore 2A. As a result,since the oil 13 flows to regions of the dynamic pressure generatinggrooves 1B and 1C sufficiently, absence of the oil film does not occur.Meanwhile, the dimensions S2, S3, S5 and S6 are set such that thedimension S2 is smaller than the dimension S6, the dimension S3 issmaller than the dimension S6 and the dimension S5 is smaller than thedimension S6, i.e., S2<S6, S3<S6 and S5<S6. By setting the dimensionsS2, S3, S5 and S6 as described above, the oil 13 does not flow out ofthe clearance of the largest dimension S6 between the small diameterportion 30A and the inner peripheral edge 25A of the cover plate 25.

In the hydrodynamic bearing device of this embodiment, a ventilationport 25B is provided on the cover plate 25. In a plane containing thecover plate 25, the ventilation port 25B is disposed so as tocircumferentially deviate by 180 degrees in FIG. 1 from a mouth of thefirst communication hole 2E opening to the upper reservoir 15. If theventilation port 25B is aligned with the mouth of the firstcommunication hole 2E, such an incident may happen that when air risingthrough the first communication hole 2E is discharged from theventilation port 25B, the oil 13 is also expelled outwardly. Thisexpulsion of the oil 13 can be prevented by circumferentially shiftingthe ventilation port 25B and the first communication hole 2E from eachother as described above. Air which has flowed out of the upper end ofthe first communication hole 2E travels circumferentially in the upperreservoir 15 along the cover plate 25 and runs outwardly when the airhas reached the ventilation port 25B.

Fourth Embodiment

FIG. 10 is a fragmentary sectional view showing a shaft 35 and thesleeve 2 of a hydrodynamic bearing device according to a fourthembodiment of the present invention. In FIG. 10, dynamic pressuregenerating grooves 35D are formed on a lower end face 35C of the shaft35. Therefore, the flange 3 of the hydrodynamic bearing device of thethird embodiment shown in FIG. 9 is not provided on the shaft 35. Otherconstructions of this hydrodynamic bearing device are substantiallysimilar to those of the hydrodynamic bearing device of FIG. 9. At an endportion of the shaft 35, on which the rotor hub 12 is mounted, the shaft35 has a small diameter portion 35A. A diameter of the inner peripheraledge 25A of the cover plate 25 is set to be larger than an outsidediameter of the small diameter portion 35A and smaller than an outsidediameter of the shaft 35. Namely, the inner peripheral edge 25A of thecover plate 25 is arranged to cover a clearance between the shaft 35 andthe bearing bore 2A of the sleeve 2. Thus, it is possible to positivelyprevent leakage of the oil 13 from the upper clearance between the shaft35 and the sleeve 2 in FIG. 10.

The dynamic pressure generating grooves 35D which may be formed on oneof the lower end face 35C of the shaft 35 and an upper face of thethrust plate 4 are formed on the lower end face 35C of the shaft 35 inFIG. 10 and confront the thrust plate 4 so as to constitute a thrustbearing with the thrust plate 4. In FIG. 10, the step portion 2D isformed on the underside of the sleeve 2. An end portion of the bearingbore 2A including the step portion 2D of the sleeve 2 is sealed by thethrust plate 4. A space between the step portion 2D and the thrust plate4 communicates with the first communication hole 2E at the lower inlet2F. The first communication hole 2E acts as a communication path forestablishing communication between the step portion 2D and the upperreservoir 15.

In the hydrodynamic bearing device of this embodiment, the dynamicpressure generating grooves 35D are provided on the lower end face 35Cof the shaft 35 without providing the flange on the shaft 35. Hence, incomparison with the foregoing embodiments, the construction is simplerand thus, is cheaper.

Also in the hydrodynamic bearing device of this embodiment, dynamicpressure generating grooves 2B and 2C formed by shallow grooves of aherringbone pattern are provided on at least one of the outer peripheralface of the shaft 35 and the inner peripheral surface of the sleeve 2(on the inner peripheral surface of the sleeve 2 in FIG. 10) and theclearance between the shaft 35 and the sleeve 2 is filled with the oil13 in the same manner as the foregoing embodiments. The upper reservoir15 is provided on the sleeve 2 in the vicinity of the upper end face ofthe sleeve 2 and communicates with a space adjacent to the lower endface 35C of the shaft 35 through the first communication hole 2E. Thus,the oil 13 circulates in a path in which the oil 13 flows from the upperreservoir 15 into the clearance between the shaft 35 and the sleeve 2and returns from the lower portion of the sleeve 2 to the upperreservoir 15 via the first communication hole 2E. Since air mixed intothe oil 13 is discharged outwardly from the ventilation port 25B of thecover plate 25 during operation of the hydrodynamic bearing device, theair in the oil 13 is eliminated and thus, absence of the oil film doesnot occur in the clearance around the shaft 35. Thus, the hydrodynamicbearing device of this embodiment can preserve high reliability for along term. Meanwhile, a disk rotating apparatus employing thehydrodynamic bearing device of this embodiment has high long-termreliability.

Also in the hydrodynamic bearing device of this embodiment, since airmixed into the oil 13 is readily discharged outwardly, absence of theoil film, which is apt to happen in the hydrodynamic bearing device, isprevented, so that long service life and high long-term reliability areobtained.

Fifth Embodiment

FIG. 11 is a fragmentary sectional view showing the shaft 35 and thesleeve 2 of a hydrodynamic bearing device according to a fifthembodiment of the present invention. In FIG. 11, the hydrodynamicbearing device of this embodiment has a construction similar to that ofthe hydrodynamic bearing device of the fourth embodiment shown in FIG.10 except that a cover plate 27 is different from the cover plate 25 ofthe fourth embodiment.

The cover plate 27 of this embodiment is shown in a perspective view ofFIG. 12 a and a sectional view of FIG. 12 b along the line XIIb-XIIb. Asshown in FIGS. 12 a and 12 b, the cover plate 27 has, on its lower face,at least one recess 27E. A boss 27H is formed at a portion of an upperface of the cover plate 27, which portion corresponds to the recess 27E.A ventilation port 27F is provided at a substantially central portion ofthe recess 27E. The cover plate 27 is attached to the sleeve 2 such thatthe recess 27E confronts the upper reservoir 15.

In this embodiment, the clearance between the cover plate 27 and theupper reservoir 15 becomes large at the recess 27E of the cover plate27. The oil 13 in the upper reservoir 15 is least likely to flow intothe large clearance below the recess 27E due to its surface tension andthus, remains in a portion of a small clearance surrounding the recess27E. Hence, since the ventilation port 27F disposed at the centralportion of the recess 27E is not covered by the oil 13, air in the upperreservoir 15 is discharged smoothly from the ventilation port 27F.

In the hydrodynamic bearing device of this embodiment, supposing that“S1” denotes a dimension of a radial clearance between the bearing bore2A of the dynamic pressure generating grooves 1B and the shaft 35, “S2”denotes a dimension of a radial clearance between an outer periphery ofthe shaft 35 and an inner periphery of the upper end portion 2H of thesleeve 2, “S3” denotes a dimension of a clearance between the coverplate 27 and an end portion of the shaft 35 and “S4” denotes a dimensionof a clearance at an outer peripheral portion of the upper reservoir 15,the dimensions S2 and S3 are set to be larger than the dimension S1,i.e., S1<S2 and S1<S3. Meanwhile, the dimension S4 is set to be largerthan the dimension S3, i.e., S4>S3. As a result, the oil 13 in the upperreservoir 15 gathers to a vicinity of an opening 27A having theclearance of the small dimension S3 due to its surface tension and then,flows into the clearance (radial bearing portion) having the smallerdimension S1 between the shaft 35 and the bearing bore 2A.

Each of the dynamic pressure generating grooves 1B has the upper groove1L and the lower groove 1M, which have the dimensions L and M from theangular portion 1K such that the upper groove 1L is larger than thelower groove 1M. Hence, the oil 13 which has flowed into the clearanceof the dimension S2 between the upper end portion 2H of the sleeve 2 andthe shaft 35 is drawn into the radial clearance of the dimension S1between the shaft 35 and the bearing bore 2A of the sleeve 2 by pumpingaction of the dynamic pressure generating grooves 1B at the time ofstart of operation of the hydrodynamic bearing device and duringoperation of the hydrodynamic bearing device. By this action, the oil 13in the upper reservoir 15 is caused to flow into the radial bearingpositively.

Air in the form of minute air bubbles is mixed into the lubricant suchas the oil 13 filled in the clearance between the sleeve 2 and the shaft35. If quantity of the air mixed into the oil 13 is large, the airbubbles are expanded by rise of internal pressure of the air bubblesupon rise of ambient temperature or the air bubbles are expanded in anenvironment of low pressure, volume of the air increases. The air whosevolume has increased enters the first communication hole 2E from thelower inlet 2F of the first communication hole 2E as indicated by air14. In FIG. 11, the air 14 proceeds further upwardly so as to enter theupper reservoir 15. The air 14 which has entered the upper reservoir 15travels circumferentially and is discharged outwardly from theventilation port 27F as shown by the arrow C when the air 14 has reachedthe recess 27E. At this time, the oil 13 is also displaced together withthe air 14 in the first communication hole 2E. However, since the oil 13which has been carried to the upper reservoir 15 is separated from theair 14, only the oil 13 remains in the upper reservoir 15 due to itssurface tension, while the air 14 is discharged from the ventilationport 27F as shown by the arrow C. Hence, the oil 13 is neither forcednor leaked out of the hydrodynamic bearing device. Therefore, thehydrodynamic bearing device can be rotated stably without beingsubjected to absence of the oil film.

In this embodiment, the shaft 35 has a diameter of 1 to 20 mm and thedimension S3 of the clearance ranges from 30 to 150 microns. Thedimension S1 of the radial clearance of the radial bearing ranges from 1to 10 microns and the first communication hole 2E has a diameter of 0.3to 1.0 mm. In the hydrodynamic bearing device having the dimensions setin the above ranges, it has been confirmed that the oil 13 issatisfactorily held in the respective clearances of the hydrodynamicbearing device and the air 14 is discharged favorably.

In FIG. 11, even if a drop impact load or vibrations are applied in adirection of the arrow G2, the oil 13 stored in the upper reservoir 15is held in the upper reservoir 15 due to its surface tension withoutflowing outwardly.

Experiments conducted by the present inventor have revealed that even ifan acceleration of 2,500 G is applied for 1 to 10 msec by setting thedimensions S2 and S3 to about 50 microns, the oil 13 does not leak androtation of the hydrodynamic bearing device can be continued by holdingthe shaft 35 and the sleeve 3 out of contact with each other.

Sixth Embodiment

FIG. 13 is a fragmentary sectional view showing the shaft 35 and thesleeve 2 of a hydrodynamic bearing device according to a sixthembodiment of the present invention. In this embodiment, a cover plate28 is different from the cover plate 27 of the hydrodynamic bearingdevice of the fifth embodiment shown in FIG. 12. Other constructions ofthis hydrodynamic bearing device are similar to those shown in FIG. 1.As shown in a perspective view of FIG. 14 a and a sectional view of FIG.14 b, the cover plate 28 of this embodiment is formed with a bulgeportion 28D by partially bulging an inner peripheral portion of theringlike cover plate 28. A recess 28E is formed on one face of the coverplate 28 opposite to the bulge portion 28D. The cover plate 28 isattached to the sleeve 2 such that the recess 28E confronts the upperend portion 2H on the end face of the sleeve 2. In an area in which therecess 28E and the end face of the sleeve 2 confront each other,clearance between the recess 28E and the end face of the sleeve 2increases by a size of the recess 28E. Thus, the oil 13 does not gatherto a vicinity of the recess 28E due to its surface tension. Therefore,air which has reached the upper reservoir 15 by way of the firstcommunication hole 2E travels in the annular upper reservoir 15circumferentially and is smoothly discharged from the recess 28E whenthe air has reached the recess 28E. Since the oil 13 proceeds to thesmall clearance due to its surface tension and does not gather to avicinity of the recess 28E having the large clearance, leakage of theoil 13 out of the recess 28E does not occur.

In this embodiment, the recess 28E can be formed by such a simpleworking in which the inner peripheral portion of the cover plate 28 isrecessed.

FIG. 15 is a perspective view showing a cover plate 40 as anotherexample of the cover plate 28 of the hydrodynamic bearing device of thisembodiment. Other constructions than the cover plate 40 are similar tothose of FIG. 13. In FIG. 15, the ringlike cover plate 40 is formed, atits inner peripheral portion, with a notch 40E. Since a part of theupper end portion 2H of the inner peripheral portion of the sleeve 2 isconnected to an outside by the notch 40E, air is discharged smoothlythrough the notch 40E. The notch 40E can be formed by performing simpleworking, for example, simultaneously with press working of the coverplate 40. Since the notch 40E can be worked easily, manufacturing costof the cover plate 40 is also reduced.

Seventh Embodiment

FIG. 16 is a fragmentary sectional view showing the shaft 35 and thesleeve 2 of a hydrodynamic bearing device according to a seventhembodiment of the present invention. FIG. 17 is a sectional view alongthe line XVII-XVII in FIG. 16. In this embodiment, a cover plate 41 isstructurally different from that of the sixth embodiment and otherconstructions are similar to those of the sixth embodiment.

The cover plate 41 of this embodiment has a disklike portion 41A and acylindrical portion 41B which are formed integrally with each other. Anupper portion of the sleeve 2 is desirably press fitted into thecylindrical portion 41B. The sleeve 2 may also be inserted into thecylindrical portion 41B so as to be attached thereto.

In the hydrodynamic bearing devices of the foregoing embodiments, theshaft 35 is formed by an iron-series material having high rigidity.Meanwhile, the sleeve 2 is formed by a copper-series material such asfree-cutting brass in which high machining accuracy is obtained easilyby its quite excellent machinability. The cover plate 41 is at leastmade of a material whose coefficient of linear expansion is smaller thanthat of the sleeve 2. For example, it is desirable that the cover plate41 is made of an iron-series material having high rigidity in the samemanner as the shaft 35.

However, if the shaft 35 and the sleeve 2 of the hydrodynamic bearingdevice of this embodiment are made of the materials referred to above,the sleeve 2 is expanded due to difference in the coefficients of linearexpansion of the respective materials when the hydrodynamic bearingdevice has reached a high temperature. Consequently, the dimension S1 ofthe radial clearance between the shaft 35 and the sleeve 2 increases asshown in FIG. 17, thereby possibly resulting in drop of pressuregenerated by the hydrodynamic bearing device and drop of rigidity of theoil film.

Therefore, in this embodiment, the shaft 35 is formed by an iron-seriesmaterial such as a ferrite-series or martensite-series stainless steelhaving a coefficient of linear expansion of 1.03×10⁻⁵/° C. and thesleeve 2 is formed by a copper alloy having a coefficient of linearexpansion of 2.05×10⁻⁵/° C. Meanwhile, the cover plate 41 is formed by amartensite-series stainless steel having a coefficient of linearexpansion of 1.03×10⁻⁵/° C. If the materials are selected as describedabove, inside diameter of the cylindrical portion 41B of the cover plate41 does not increase so much due to small amount of expansion of thecylindrical portion 41B. On the other hand, the sleeve 2 is made of thematerial having the coefficient of linear expansion larger than that ofthe cover plate 41. Hence, when temperature rises, amount of expansionof the outside diameter of the sleeve 2 is larger than that of an insidediameter of the cylindrical portion 41B of the cover plate 41. However,since the less expandable cylindrical portion 41B of the cover plate 41grasps the outside diameter of the sleeve 2, thermal expansion of theoutside diameter of the sleeve 2 is restrained by the cylindricalportion 41B of the cover plate 41. Namely, the cylindrical portion 41Bof the cover plate 41 is capable of restricting expansion of the insideand outside diameters of the sleeve 2 by applying pressure to the outerperiphery of the sleeve 2.

In this embodiment, even at high temperatures, amount of thermalexpansion of the inside and outside diameters of the sleeve 2 is smalland is not so different from that of the shaft 35. Thus, it can bearranged that the dimension S1 of the radial clearance of the radialbearing does not change greatly upon changes in temperature. As aresult, changes of performance of the hydrodynamic bearing device withtemperature are restrained. Meanwhile, since the cylindrical portion 41Bof the cover plate 41 is fixed to the outer periphery of the sleeve 2,the cover plate 41 is securely mounted on the sleeve 2 and thus, thereis no risk that the shaft 35 is detached from the sleeve 2.

In this embodiment, since air mixed into the oil 13 of the hydrodynamicbearing device is readily discharged, absence of the oil film, which isoften associated with prior art bearings, is prevented and it ispossible to minimize change of the radial clearance between the shaft 35and the sleeve 2 upon changes in temperature of the hydrodynamic bearingdevice. Thus, it is possible to materialize the hydrodynamic bearingdevice operating with high precision for a long service life even in ause environment having changes in temperature. By employing thishydrodynamic bearing device, it is possible to obtain the disk rotatingapparatus operating with high precision for a service life.

INDUSTRIAL APPLICABILITY

The present invention is applicable to not only the hydrodynamic bearingdevice which is highly reliable for a long service life by preventingleakage of the lubricant but the disk rotating apparatus employing thishydrodynamic bearing device.

1. A hydrodynamic bearing device comprising: a sleeve having a bearingbore into which a shaft is rotatably inserted; a cover plate which isprovided such that a reservoir for storing lubricant and air is definedin the vicinity of one end portion of the bearing bore; a substantiallydisklike flange which is secured to one end portion of the shaft and hasone face confronting one end face of the sleeve in the vicinity of theother end portion of the bearing bore; a thrust plate which is providedso as to confront the other face of the flange and seal a regionincluding the one end face of the sleeve; and at least one communicationpath extending from the reservoir of the cover plate toward an outersurface of the flange; wherein first and second dynamic pressuregenerating grooves of a herringbone pattern are arranged in a directionalong an axis of the shaft on at least one of an inner peripheralsurface of the bearing bore of the sleeve and an outer peripheralsurface of the shaft; wherein a third dynamic pressure generating grooveof a spiral pattern is provided on at least one of opposed faces of theflange and the thrust plate and a fourth dynamic pressure generatinggroove of a herringbone pattern is provided on at least one of the oneface of the flange and the one end face of the sleeve; whereinclearances between the shaft and the sleeve and between the flange andthe thrust plate including the first, second, third and fourth dynamicpressure generating grooves are filled with lubricant; wherein one ofthe sleeve and the shaft is mounted on a stationary base and the otherof the sleeve and the shaft is mounted on a rotary member; and whereinwhen “S1” denotes a dimension of a radial clearance between the shaftand the sleeve at the time the axis of the shaft is held in alignmentwith a central axis of the bearing bore, “S2” denotes a dimension of aclearance between the shaft and an inner periphery of the cover plateand “S3” denotes a dimension of clearance between an inner peripheralportion of the cover plate and the other end face of the sleeve, andwherein the dimension S1 is made smaller than the dimensions S2 and S3.2. A hydrodynamic bearing device comprising: a sleeve having a bearingbore into which a shaft is rotatably inserted; a cover plate which isprovided such that a reservoir for storing lubricant and air is definedin the vicinity of one end portion of the bearing bore; a substantiallydisklike flange which is secured to one end portion of the shaft and hasone face confronting one end face of the sleeve in the vicinity of theother end portion of the bearing bore; a thrust plate which is providedso as to confront the other face of the flange and seal a regionincluding the one end face of the sleeve; and at least one communicationpath extending from the reservoir of the cover plate toward an outersurface of the flange; wherein first and second dynamic pressuregenerating grooves of a herringbone pattern are arranged in a directionalong an axis of the shaft on at least one of an inner peripheralsurface of the bearing bore of the sleeve and an outer peripheralsurface of the shaft; wherein a third dynamic pressure generating grooveof a spiral pattern is provided on at least one of opposed faces of theflange and the thrust plate and a fourth dynamic pressure generatinggroove of a spiral pattern is provided on at least one of the one faceof the flange and the one end face of the sleeve; wherein clearancesbetween the shaft and the sleeve and between the flange and the thrustplate including the first, second, third and fourth dynamic pressuregenerating grooves are filled with lubricant; wherein one of the sleeveand the shaft is mounted on a stationary base and the other of thesleeve and the shaft is mounted on a rotary member; and wherein when“S1” denotes a dimension of a radial clearance between the shaft and thesleeve at the time the axis of the shaft is held in alignment with acentral axis of the bearing bore, “S2” denotes a dimension of aclearance between the shaft and an inner periphery of the cover plateand “S3” denotes a dimension of clearance between an inner peripheralportion of the cover plate and the other end face of the sleeve, andwherein the dimension S1 is made smaller than the dimensions S2 and S3.3. The hydrodynamic bearing device as claimed in claim 1 or claim 2,wherein when “S4” denotes a dimension of a clearance between an outerperipheral portion of the cover plate and the other end face of thesleeve, and the dimension S3 is made smaller than the dimension S4.
 4. Ahydrodynamic bearing device comprising: a shaft which has, at its oneend portion, a thrust bearing face perpendicular to an axis of theshaft; a sleeve having a bearing bore into which the shaft is rotatablyinserted; a cover plate which is provided such that a reservoir forstoring lubricant and air is defined in the vicinity of one end portionof the bearing bore; a thrust plate which is provided so as to seal theother end portion of the bearing bore and confront the thrust bearingface of the shaft; and at least one communication path for establishingcommunication between the reservoir and a region of the other endportion of the bearing bore; wherein first and second dynamic pressuregenerating grooves of a herringbone pattern are arranged in a directionalong the axis of the shaft on at least one of an inner peripheralsurface of the bearing bore of the sleeve and an outer peripheralsurface of the shaft; wherein a third dynamic pressure generating grooveof a spiral pattern is provided on at least one of the thrust bearingface and one face of the thrust plate confronting the thrust bearingface; wherein clearances between the shaft and the sleeve and betweenthe thrust bearing face and the thrust plate including the first, secondand third dynamic pressure generating grooves are filled with lubricant;wherein one of the sleeve and the shaft is mounted on a stationary baseand the other of the sleeve and the shaft is mounted on a rotary member;and wherein when “S1” denotes a dimension of a radial clearance betweenthe shaft and the sleeve at the time the axis of the shaft held inalignment with a central axis of the bearing bore, “S2” denotes adimension of a clearance between the shaft and an inner periphery of thecover plate and “S3” denotes a dimension of clearance between an innerperipheral portion of the cover plate and the an end face of the sleeve,and wherein the dimension S1 is made smaller than the dimension S2. 5.The hydrodynamic bearing device as claimed in claim 4, wherein when “S4”denotes a dimension of a clearance between an outer peripheral portionof the cover plate and the end face of the sleeve, the dimension S3 ismade smaller than the dimension S4.
 6. The hydrodynamic bearing deviceas claimed in claim 5, wherein when “S6” denotes a dimension of aclearance between an outer periphery of a small diameter portion of theshaft and the inner periphery of the cover plate, the dimension S3 ismade smaller than the dimension S6; and Wherein the shaft has, at theother end portion, a small diameter portion having a diameter smallerthan that of the shaft, while an inside diameter of the cover plate islarger than the diameter of the small diameter portion and smaller thanthe diameter of the shaft.
 7. The hydrodynamic bearing device as claimedin claims 1, 2 or 4, wherein at least one ventilation port forestablishing communication between the reservoir and exterior is formedon the cover plate so as to circumferentially deviate from thecommunication path.
 8. The hydrodynamic bearing device as claimed inclaim 7, wherein by bulging one circumferential portion of the coverplate, the ventilation port has a recess on one face of the cover plateadjacent to the reservoir.
 9. A disk rotating apparatus including thehydrodynamic bearing device of one of claims 1, 2-8, in which a rotorhub having a rotor magnet is mounted on the other end portion of theshaft and at least one disk for recording and reproduction is mounted onthe rotor hub by a damper.